Question: Nadia is 5 times as old as Kevin and is also 40 years older than Kevin. How old is Kevin?
Answer: We can use the given information to write down two equations that describe the ages of Nadia and Kevin. Let Nadia's current age be $n$ and Kevin's current age be $k$ $n = 5k$ $n = k + 40$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $k$ , and both of our equations have $n$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $5k$ $-$ $ (k + 40)$ which combines the information about $k$ from both of our original equations. Solving for $k$ , we get: $4 k = 40$ $k = 10$.